The present invention relates in general to cardiac surgery, and, more specifically, to the heating and cooling of blood or other fluids delivered to a patient during cardiac bypass surgery.
Heating and cooling devices are an important part of blood perfusion systems used during cardiac surgery. During surgery, blood is cooled in a bypass circuit to induce hypothermia to protect the organs. A separate cardioplegia circuit typically provides a dedicated flow of cooled solution directly to the heart, at least periodically. When the surgery has been completed, the blood and/or other fluids flowing in the two circuits are heated prior to the patient waking from anesthesia. During various circumstances that may arise during operation of the blood perfusion system, it becomes desirable not only to heat both circuits or cool both circuits simultaneously, but also to cool one circuit while the other is heating or to deactivate one circuit while the other is either heating or cooling.
Conduits carrying the blood and/or cardioplegia in each circuit pass through respective heat exchangers. Water (or other heat exchange fluid) in the two respective heater/cooler circuits is pumped through passages in the heat exchangers for adding heat to or removing heat from the blood/cardioplegia as necessary. An integrated heater/cooler unit having an integrated controller and an integrated power supply usually includes a single ice-bath compartment for selectably cooling the water in both water circuits and a pair of heating devices for selectably heating the water in the two circuits independently.
In view of electrical safety standards and the desire to power a dual heater/cooler unit from a single conventional outlet in an operating room, it is necessary to ensure that the current drawn from the outlet stays safely within a maximum limit. The significant power-consuming elements of the unit are the controller electronics (e.g., microcontroller, display, and other related circuitry), two water-circulating pumps, and two heaters. The cooling function does not consume power other than that to operate the controls and pumps since ice is used as a source of cooling. The maximum current draw occurs when both heaters operate simultaneously (i.e., both the arterial and the cardioplegia patient circuits are being heated and both pumps are operating).
U.S. Pat. No. 6,423,268, issued to King et al., discloses a dual heater wherein circuitry is provided to prevent the first and second heaters from being activated simultaneously. Furthermore, a heater is not activated until after a delay from the time when the other heater is deactivated. Thus, when heating is needed in both fluid circuits, King et al. alternately activates each of the two heaters with a suitable delay time between activations so that instantaneous switching is avoided. By driving the two heaters in a complementary fashion (separated by an off time), both circuits are heated without exceeding the available current. However, a relatively complicated and expensive heater control system including multiple relays is required to obtain the necessary delays. It would be desirable to limit the maximum current draw without such complications or expense.
The relay switching system as implemented in King et al has additional disadvantages. A significant in-rush current may flow when a heater is activated. The in-rush current may lead to the tripping of a circuit breaker and/or excessive generation of heat in the switching device. Due to inherent variations in the timing provided by different relay devices (even of the same manufacturer and part number), significant timing errors may occur when activating the heaters. The errors can become large enough to inadvertently cause the heaters to be activated simultaneously, resulting in an excessive current draw. It would be desirable to eliminate in-rush current and to provide a more precise and robust control of heater currents in order to gain performance benefits and to avoid simultaneous activation errors.